1. Field of the Invention
The present invention generally relates to an optical disk drive, and more particularly, to an optical disk drive that determines write tracking offset automatically.
2. Description of the Related Art
Grooves and lands, the portion between two grooves adjacent to each other, are formed on a write-once type optical disk such as a compact disk-recordable (CD-R) or a digital versatile disk-recordable (DVD-R) in advance. An optical disk drive designates either the grooves or the lands as the tracks and forms pits indicating information by applying a laser beam on the track. The tracking servo unit of the optical disk drive controls the position in a radial direction of the optical axis of the laser beam to trace the center of the track.
The groove is slightly wobbled at a center frequency of 22.05 kHz in the radial direction and contains address information, that is, time information called an absolute time in pre-groove (ATIP) having a maximum divergence of +/−1 kHz multiplexed to the center frequency by FSK modulation that is used in a recording operation. Since the amplitude of the wobble signal is small, it does not affect the tracking servo. Because the frequency of the wobble signal is not in the tracking servo frequency range, the laser beam axis traces the center of the track on average.
The optical disk drive receives the laser beam reflected by the optical disk with a photo detector having a light detecting face divided into two portions in the radial direction of the optical disk, and detects the wobble signal by differentially amplifying the RF signals received by the two portion of the photo detector. The optical disk drive controls the spindle motor rotation based on the carrier frequency of the wobble signal, 22.05 kHz and rotates the optical disk at a predetermined rotative speed. The optical disk drive further demodulates the wobble signal to obtain the address information.
In a read operation, the optical disk drive controls the laser beam axis and traces the center of the track. The optical disk drive applies a laser beam on the pits formed on the track of the optical disk, and receives the laser beam reflected by the optical disk with the photo detector as described above. By adding two optoelectronic signals received by the corresponding portions of the photo detector, the optical disk drive eliminates the wobble signal and detects the read signal that has been recorded.
FIG. 13 is a schematic drawing showing the format of information recorded in a CD-R disk, a write once type optical disk. In the direction from an inner radius to an outer radius, a power calibration area (PCA) where the optimum write power is determined and an information area are provided. The information area consists of a program memory area (PMA) for the temporary storage of signal recording information, a lead-in area, a user's data area, and a lead-out area.
In order to determine the optimum write power of the laser beam, a CD-R drive performs an optimum power control (OPC) operation before it starts an actual write operation. In the OPC operation, the CD-R drive measures the recording properties of a CD-R disk by recording and reproducing test signals in the power calibration area located in a predetermined position of the CD-R disk. The power calibration area includes a test area for 100 test recordings, partitions P001–P100. Each partition consists of 15 frames F01–F15. The test signal is recorded in 15 frames at 15 different write powers that increases in 15 steps, each frame being recorded by a corresponding write power, and is reproduced. The optimum power is determined based on the peak value and the bottom value of the reproduced test signal. Among the 15 different write powers, the power that best fits the disk is determined as the optimum power. The OPC operation is necessary to compensate for the difference in the recording properties of disks manufactured by different manufacturers.
FIG. 14 depicts an AC coupled radio frequency (RF) signal reproduced from a CD-R disk. The envelopes, a peak value (P), and a bottom value (B) of the signal are also shown in FIG. 14. The AC coupled RF signal is the AC component signal resulting from filtering out a DC component from the reproduced read signal. In the OPC operation, the test signal is recorded in 15 frames in the test area at 15-stepped write power, each frame being recorded by a corresponding write power of a laser diode. Each frame is recorded by a write power corresponding to the frame. The write power is increased, frame by frame, in 15 steps from the minimum power to the maximum. The peak value and the bottom value of the RF signal are detected as shown in FIG. 14.
The peak value (P) is positive and the bottom value (B) is negative. A characteristic value β is defined as β=(P+B)/(P−B). If the absolute value of the peak value (P) is equal to the absolute value of the bottom value (B), the peak point and the bottom point are positioned at the same distance from the 0 V line but in the opposite direction, and β=0 (P+B=0). The characteristic value β changes as the write power increases. If the characteristic value β exceeds a predetermined value (0.04, for example), the write power is determined as the optimum write power. Then, write operations are performed at the optimum write power.
Because a read power and a write power differ, the laser beam axes corresponding to the read power and the write power are often slightly different from each other because the laser beam axis depends on a power level. FIG. 15 shows the difference in the optical axes between the read power and the write power. The laser beams corresponding to the read power and the write power are described with a solid line and a dotted line, respectively. The difference in the angle of the optical axes is indicated by “θ”. If the optical axis slides in the direction of a groove width, that is, the radial direction of the optical disk, pits that are formed with the write power are formed off the track center as shown in FIG. 16, since the tracking servo is performed with the read power laser beam.
In the case that the groove is designated as the track, because the groove is wobbled, the write power laser beam axis repeatedly moves to the groove edge and to the groove center at a frequency of the wobble. When the laser beam comes to the groove center, pits are formed in a normal manner, but if the laser beam traces far from the groove center, pits are formed insufficiently since the groove edge affects the pits. Accordingly, the read signal is affected at the wobble frequency.
Because the tracking error signal is generated based on the pits and the grooves in a read operation, the laser beam passes through substantially on the center of the pits. However, the reproduced RF signal is modulated and/or distorted at the wobble frequency due to the sliding of the pits during a write operation. The effect of the sliding of the pits during the write operation becomes greater as the frequency of the write signal increases. As to the CD-R, the write signal having a cycle time of 3×T (T=230 nsec, that is, a cycle time of the frequency 4.32 MHz corresponding to a single speed) is most affected. FIG. 17 is waveform of a reproduced RF signal.
If the envelopes of the reproduced RF signal are modulated by the wobble signal component, the modulation causes jitter (chattering noise) when the reproduced RF signal is converted into a binary signal. The binary signal outputted by the comparator contains the wobble signal component.